Application of the quartz crystal microbalance to measurement of the concentration of electrolyte in lead/acid batteries

The density of the electrolyte in a lead/acid battery may change by 10 to 20% during the discharge/charge cycle and the value within this range can be used as an indication of the fraction of available energy stored in the battery. We describe the application of a quartz crystal microbalance to the determination of the density of sulfuric acid solutions over a range of temperatures and during the discharge of a typical lead/acid battery. The sensor response varies linearly with temperature, and in a quadratic manner with the density of the solution. Good agreement with the theoretically predicted behaviour of thickness shear mode oscillators immersed in liquids was observed. The sensor performed well in the laboratory experiments under artificial conditions. By contrast, initial studies using the sensor in a lead/acid battery operating under real conditions revealed that stray capacitances can influence the stability of the oscillator.